1. Field of the Invention
The present invention relates to the field of manufacturing technology of liquid crystal displays, and in particular to a rubbing device and a rubbing method.
2. The Related Arts
A rubbing process for alignment is an important step of a manufacturing process of a thin-film transistor liquid crystal display and is carried out before a boxing operation and is generally used in a twisted nematic (TN) mode, an in-plane switching (IPS) mode, and a fringe field switching (FFS) mode as an operation for alignment applied to a TFT (Thin-Film Transistor) array substrate or a CF (Color Filter) substrate that has already been coated with an aligning agent.
A rubbing process, specifically speaking, is a process of rubbing a surface of an alignment film in a fixed direction by using a soft rubbing fabric so as to form grooves that force liquid crystal to align in a predetermined direction. The rubbing process also generates a pre-tilt angle to ensure the liquid crystal to stand in the same direction. Thus, the rubbing process is a vital part of a manufacturing process of a liquid crystal display.
A rubbing device that is employed in the rubbing process is generally composed of a rubbing roller, a piece of fabric, an elevation arm, and a substrate carrier. The rubbing fabric is set in advance on a surface of the rubbing roller and the elevation arm controls a vertical distance between the rubbing roller and the substrate carrier. By adjusting the vertical distance, the rubbing fabric set on the rubbing roller can be put into contact with the substrate carried on the substrate carrier with a proper aligning pressure in order to carry out rubbing. Generally, parameters that affect the rubbing process include a moving speed of the substrate carrier, the radius of the aligning roller, a rotational speed of the aligning roller, times of aligning, and the aligning pressure, among which the rotational speed and the moving speed are process control parameters and the selection of the material of the rubbing fabric is closely related to the yield rate of the alignment-completed substrate. The material of the rubbing fabric is generally divided into cotton, artificial fiber (such as Rayon), and Nylon.
Heretofore, before the rubbing process is applied to an alignment film, the rubbing fabric is first attached to the surface of the rubbing roller. The rubbing roller with the rubbing fabric attached to the surface thereof is then subjected to pre-rubbing of the rubbing fabric on the surface of the rubbing roller in a rubbing device that comprises a pre-rubbing substrate placed therein so as to have fuzzes of the rubbing fabric aligning in the same direction for being used in the rubbing process to form grooves that aligns liquid crystal in a predetermined direction.
Rubbing is a vital link of the current manufacturing technology of LCD (Liquid Crystal Display). However, it is inevitable that rubbing will generate some tiny dust powders (namely PI (Polyimide) chips and flakes). Such dust powders may causes defects of fragmented bright spots and mura. Thus, it is a challenge for panel workshops to reduce the dust powder induced by the rubbing process.
Referring to FIG. 1, a schematic view is given to illustrate the principle of generating dust powders in a conventional rubbing process. A substrate carrier 11 undergoes translation in the direction indicated by the arrow. A substrate 12, which is to be subjected to alignment, is placed on the substrate carrier 11 to undergo the translation with the substrate carrier 11 in a direction toward a rubbing roller 13. The rubbing roller 13 comprises a piece of rubbing fabric 14 attached to a surface thereof. The rubbing fabric 14 rubs a surface of the substrate 12 and PI chips 15 are generated. Due to a height difference between the substrate carrier 11 and the substrate 12, the PI chips 15 on the rubbing roller 13 will get accumulated in the located circled by phantom lines in FIG. 1 and may have a great chance to be rebounded to the top of the substrate 12, leading to defects of fragmented bright spots and mura.
Referring to FIG. 2, photos of scanning electron microscopes are given to show various types of defect generated in a conventional rubbing process. A substrate carrier 21 and a substrate 22 placed thereon are moved in a direction indicated the arrow toward an aligning roller 23. The aligning roller 23 comprises a piece of rubbing fabric 24 attached to a surface thereof. After the rubbing process is completed, a portion 221 of the substrate 22 is subjected to photographing by means of a scanning electron microscope and various types of defect generated in the conventional rubbing process can be obtained and displayed, such as PI settling, irregular scratching, PI scratching, and ITO (Indium Tin Oxide) collapsing.
A process of generation of defects of fragmented bright spot and mura will be described with reference to FIG. 2: (1) When the rubbing roller 23 is rotated in a clockwise direction with respect to the translation direction of the substrate carrier 21 and the substrate 22, due to cliff effect, PI chips on the right side of the rubbing roller 23 will be carried by the rubbing fabric 24 to get accumulated at the left side; (2) due to slit effect, the carried PI chips will gather in slits of the rubbing fabric 24 and the substrate 22 or will be carried away later; (3) due to distance effect, the carried PI chips are moved backed to a different site on the substrate 22 without being subject to alignment; (4) PI chips that are left in the AA zone (an effective displaying zone) cause fragment bright spots; and (5) PI chips that are away from the rubbing roller 23 form mura.